The use of antiscatter grids in radiation, in particular X-ray, diagnosis is the most widely used and recognized method for reducing the proportion of scattered radiation in the imaging radiation, the primary radiation, and for improving the contrast of the radiation image recording. The grids that are mainly used nowadays are focused linear grids. These linear grids include absorber lamellae, generally lead lamellae, embedded in a carrier material, generally paper or plastic layers.
For focusing purposes, the absorber lamellae are arranged upright or inclined with respect to the vertical in such a way that the diverging primary radiation can pass through between the lamellae, but the scattered radiation is blocked (grid focusing). Each grid is focused with respect to a specific, defined distance from the focus of the beam source. The inclination of the absorber lamellae corresponds to the divergence of the primary beam cone at a specific distance with respect to which the grid is focused.
Any deviation from the focusing distance leads to a dose decrease in the primary radiation primarily in the image edge regions. This is due to the fact that, in the case of a deviation from the focusing distance, the clear width between the lamellae decreases and, consequently, more primary radiation is absorbed by way of the absorber lamellae, the absorption increasing with increasing deviation from the focusing distance, that is to say with increasing defocusing.
The distance tolerances which are specified for each grid and within which a defocusing still leads to acceptable, diagnostically meaningful images are based on a dose decrease of 40% at the image receiver edge as seen from the grid center (IEC/DIN 60627). In this case, the decrease is given not only by the different transmittivity of the grid in the case of nonfocusing, but also by the outwardly decreasing dose (square law of distance). The distance tolerance range used under these preconditions is primarily determined by the shaft ratio “R”, that is to say the ratio of the width of the shaft between two lamellae to the height of the lamellae.
In the case of the digital radiation image receivers that are increasingly being used, e.g. in the form of solid-state detectors or flat detectors, use is made of antiscatter grids having a significantly higher number of lines (of e.g. 80 lines/cm) compared with the grids used e.g. in the case of film systems. In order to obtain the same selectivity (scattered radiation suppression) as in the case of the moving grids used in conventional film radiography (shaft ratio R=8 or 12), higher shaft ratios (e.g. R=15) are used in the case of grids having a high number of lines.
It is disadvantageous when using such grids in connection with digital image detectors, however, that the distance tolerance range, that is to say the range within which a defocusing which still leads to acceptable images may be given, is significantly limited compared with the e.g. moving grids with lower shaft ratios in customary film systems. This limited distance tolerance range demands a consistent changing of the grids in the event of a changing film-focus distance, that is to say the distance of the focus of the beam source from the solid-state image detector.
However, changing the grid is time-consuming and does not permit a continuous workflow in the context of examining patients. Furthermore, it is necessary to keep in each case different grids which are focused with respect to different film-focus distances, in order, by way of example, to be able to cover a customary distance range of 115 cm to 180 cm.